Method for leak testing and leak testing apparatus

ABSTRACT

For leak testing closed containers ( 9 ) which are filled with a filling product containing at least one liquid component the container is introduced in a test cavity ( 1 ) which is evacuated at least down to vapor pressure of that liquid component. The pressure in the surrounding of the container ( 9 ) and thus within test cavity ( 1 ) is monitored. Monitoring is performed by a vacuum pressure sensor ( 7 ), whereas lowering pressure surrounding the container ( 9 ) is performed by a vacuum pump ( 5 ). Leakage is detected by monitoring a pressure change in the surrounding of the container which is due to evaporation of liquid emerging from a leak and being evaporated in the low pressure surrounding.

FIELD OF INVENTION

[0001] The present invention is generically directed on a technique forleak testing closed and filled containers, whereby the filling materialcomprises at least one liquid component.

BACKGROUND

[0002] Leak testing techniques according to which closed containers areintroduced in a test cavity which, after having sealingly been closed,is lowered in pressure by a suctioning pump are known. If the containeris not leaking, then once a predetermined pressure has been reached inthe test cavity and thus in the surrounding of a container to be tested,this pressure will be kept substantially constant. If a leak is providedin an area of the container, wherein air is entrapped, a flow of air outof the container will lead to a rise of the surrounding pressure. If aleak is present in the area of the container where filling good isentrapped, the question whether such leak will lead to a significantrise of the surrounding pressure is largely dependent on the kind offilling good as of its viscosity, whether solid particles are present inthe filling good and, obviously, on the largeness of the leak.

[0003] Different approaches have become known to accurately detect leaksat such product-filled containers, irrespective whether the leak ispresent in an air entrapping container area or in a container areacovered with filling good. One such approach which is the topic of theco-pending European patent application EP-A-0 791 814 and the U.S.patent application Ser. No. 08/862 993 proposes to provide an impedancemeasurement, specifically a resistance measurement, just adjacent to theouter wall of the container by means of an electrode arrangement: Assoon as liquid emerges from a leak it will contact a respective pair ofimpedance measuring electrodes and lead to a significant change ofimpedance measured between such electrodes.

[0004] Nevertheless, such an approach necessitates considerableadditional expenditure with respect to provision of the impedancemeasuring arrangement in each test cavity, especially of a multi-cavityin-line inspection machine and does not enable detection of very smallleaks far below of one micron and largely independent from containershape and kind of filling good.

OBJECT OF THE INVENTION

[0005] It is a primary object of the present invention to provide aleakage test method and apparatus, which may be applied to a is verylarge scale of different containers and of different filling goods,provided at least one component thereof being liquid.

[0006] It is a further object of the present invention to provide suchmethod and apparatus which are rather inexpensive with respect toelectronic and further equipment, and which thus allow for very economictesting.

[0007] It is still further an object of the present invention to providesuch method and apparatus which have a short measuring cycle andnevertheless a very high measuring accuracy.

SUMMARY OF THE INVENTION

[0008] These objects are realised by the testing method for leak testingat least one closed and filled container, whereby the content of thecontainer comprises at least one liquid component and wherein a pressuredifference is applied across at least a part of the wall of thecontainer which part is to be leak tested and wherein the appliedpressure difference is directed towards the surrounding of the containerand wherein further the pressure in the surrounding of the container ismonitored as a leak indicative signal which is characterised by the factthat the pressure difference is established by lowering the pressure inthe surrounding of the container at least to a value which accords tothe vapour pressure of the at least one liquid component of the fillingproduct of the container to be tested.

[0009] The present invention departs from the recognition that if acontainer is leaking and liquid is drawn by the lower surroundingpressure to its outside this will—at a constant volume of thesurrounding—lead to evaporation of the liquid as soon as the surroundingpressure reaches its vapour pressure. This leads to a significant changein surrounding pressure compared with the surrounding pressure whichwould establish at the same measuring conditions but with an unleakingcontainer.

[0010] Monitoring the pressure in a test cavity containing thecontainer, once vapour pressure of the possibly leaking liquid isreached reveals as being a very accurate technique for leak testing. Ithas been noted that by such a technique leak detection of containerswith a very large spectrum of filling products may accurately beperformed and that leaks at present moment down to 0.02 μm areaccurately detectable.

[0011] Further, it has been noted that the volume of the test cavity isuncritical, so that by the inventive technique it becomes possible tosimultaneously test batches of containers, thereby accurately detectingif one container of such a container batch is leaking.

[0012] As soon as the pressure surrounding a leaking container islowered with respect to its interior pressure, some of the liquid issuctioned out of the container and as soon as the surrounding pressurereaches vapour pressure it starts to evaporate. As at a constant volumeof the surrounding area of the container evaporation of the liquid leadsto increase of pressure and the pump lowering the surrounding pressuremust now remove vapour of the liquid too, significant measurements maybe done especially after the surrounding pressure of the containerbecomes lower than the said vapour pressure. Nevertheless, it ispreferred to provide pumping abilities which may evacuate thesurrounding of the container to be tested to a significantly lower valuethan said vapour pressure, namely by at least two, preferably even by atleast three decades.

[0013] As a leak-significant pressure change may be detected as soon asone of possibly several liquid components of the filling good starts toevaporate—in the case the content of the container contains more thanone liquid component—it is recommended to select the vapour pressure ofthat component of the several liquid components which is the higher andto lower the pressure of the surrounding of the container at least tothat vapour pressure value.

[0014] Although and as well known vapour pressure is a function oftemperature and thus it might be advantageous in some cases e.g. to heatthe surrounding of the container to a predetermined temperature so as tosettle the relevant vapour pressure for a predetermined liquid, theinventive method and apparatus becomes significantly less complex if thetest is performed at room temperature, and thus the vapour pressure tobe at least reached is considered at room temperature, i.e. around 20°C.

[0015] Further, a very accurate leak detection becomes possible if thesurrounding pressure of the container is measured at two subsequentpoints in time, whereby we understand under “point” that interval oftime necessary for accurately measuring the prevailing pressure.Although it is absolutely possible to realise leak detection by applyingthe pumping action of the evacuating pump to the surrounding of thecontainer and then by measuring the resulting surrounding absolutepressure after a predetermined time span, the said measuring of thesurrounding pressure at two specific points in time allows to use thefirst value measured as a reference value and then to form thedifference of the second value measured with respect to the referencevalue. There is thereby realised a pressure difference measurementinstead of an absolute pressure measurement. More specifically, thefirst pressure signal which is measured at the first point in time isstored as an electric signal, then, after having measured the secondpressure value, a difference is formed between the first value (stillstored) and the second value.

[0016] The PCT patent application No. WO94/05991 with its US counterpartNo. U.S. Pat. No. 5,239,859, assigned to the same applicant as thepresent invention, describes a method and apparatus for very accuratelyoffset-compensated pressure difference measurement. In a preferred modeof operating the method according to the present invention as well as ofrealising the inventive apparatus, that pressure difference measuringtechnique and apparatus are used. Therefore, the WO94/05991 or therespective U.S. Pat. No. 5,239,859 are fully incorporated by referencein this present disclosure, although, and as will be seen most importantfeatures are specifically described also in this present application.

[0017] Because it is largely uncritical how big the surrounding volumeof a test cavity for the container is, with respect to the volume of thecontainer to be tested, the inventive method and apparatus reveals tohave a further significant advantage:

[0018] If the wall of the at least one container to be tested withstandsthe pressure difference between container internal pressure (normallyambient pressure) and lowered surrounding pressure, such a container maysimply be introduced in the test cavity forming the surrounding, largelyirrespective how big such container is with respect to the test cavity.Nevertheless, a highly accurate indication of leakage will inventivelybe gained. Therefore, one and the same test cavity may be used for alarge number of differently sized and different-volume containers. Thisresults in a further advantage in that batches of more than one, even ofa multitude of containers, may be introduced in one test cavity formingthe surrounding and although one single container occupying only a smallpercentage of the overall cavity volume, an accurate leak indicationwill be detected if even only one of the batch-containers is leakinginto the surrounding atmosphere.

[0019] A further significant advantage of the present invention is thefollowing:

[0020] Sometimes the filled containers are not completely filled, butthere is some amount of air entrapped in the closed container. If a leakis present in that area of such a container, which is adjacent toentrapped air or gas, by lowering the surrounding pressure, such airwill be suctioned through the leak out of the container. With thepressure of the entrapped air in the container becoming progressivelylower, there will also start vaporisation of the liquid component withinthe container and such vapour will also leave through the leak. Both,namely first the air leaving through the leak, then vapour leavingthrough the leak, will enlarge the surrounding pressure so that a leakin an entrapped air region of the container will lead to a change in thesurrounding pressure, i.e. to rising of said pressure, as if the leakwas in the liquid content covered area of the container wall. Thus, byproperly setting a threshold value for leak detection according to thesmallest still tolerated pressure change in the surrounding, it becomesuncritical whether such leak is present at an air-covered container areaor at a content-covered container area.

[0021] If one and the same leak at an air-entrapped area of thecontainer leads to a smaller pressure change in the surrounding, thanthe same leak would generate if situated at a liquid-covered containerarea, it is such a pressure change which will govern setting of athreshold value to detect whether a container is leaky or not. If,inversely, one and the same leak in a liquid-covered area would resultin a smaller pressure change in the surrounding than such leak in anair-contacted wall area, then it is again that smaller pressure changewhich governs the threshold setting for detecting leaking/not leakingcontainers.

[0022] If a container under test is largely leaky, lowering of thesurrounding pressure should be stopped as soon as such leaking isdetected so as to prevent the content of the container to spoil theinterior of the test cavity or, generally spoken, the surrounding of thecontainer and possibly even the pumping arrangement more than absolutelynecessary. This is realised either by monitoring whether the pumpingaction results in a predetermined lowering of surrounding pressure ornot or one may detect spreading of content of the container into itssurrounding by means of an impedance, thereby preferably a DC resistancemeasurement in the surrounding of the container just adjacent to thewall of the container which is to be tested. This is realised byproviding an electrode arrangement in said adjacent surrounding and allaround at least that part of the container to be tested. As soon asfilling content of the container is suctioned to its outer wall, theelectrode arrangement will be bridged by such content, leading abruptlyto an indicative impedance change which, after having been detected, isused to stop further pressure lowering at the surrounding of thecontainer.

[0023] This latter technique of rapidly detecting large leaks is appliedespecially to containers where it is necessary to snugly encapsulatethem in the test cavity because their walls would not stand the pressuredifference applied. In such a case the electrode arrangement forimpedance measurement may be incorporated along the inner wall of thetest cavity, which snugly fits with the at least one container. If suchcontainer is to be tested and therefore the test cavity snugly fits itsshape, nevertheless a continuous volume is maintained between the outerwall of the container and the wall of the test cavity for defining thesurrounding of the container by providing a sustaining grid or meshinlay or preferably by roughening the interior wall of the test cavityso that a multitude of micro-embossments of the test cavity wall sustainthe container wall and prevent it from further outward bowing due to theapplied pressure difference. Thereby, the intercommunicating spacebetween such embossments defines for the surrounding space of thecontainer.

[0024] Once the container in a test cavity, defining for itssurrounding, has been detected as being leaky, it is probable that suchtest cavity will be contaminated by some of the container's content.Then, such cavity is cleaned after the leaky container has been removed,be it by evacuation and/or flushing with a flushing gas, preferablynitrogen, be it by heating or by combining these techniques, e.g. by aheated flushing gas.

[0025] If the inventive method or apparatus is applied for in-linetesting containers and thus two or more of the inventive methods and ofthe respective apparatus are operated in parallel on a set of containersand one of such containers is detected to be leaky, then the respectivetest cavity defining for its surrounding is not anymore filled with acontainer at the next measuring cycle, but is kept empty, using thatcycle during which the other cavities are in testing condition forcleaning and reconditioning the probably contaminated cavity. Further,it is proposed in some cases to accelerate squeezing-out of liquid, if aleak is present, by mechanically biasing the wall of the containerinwardly, thus rising its interior pressure over atmospheric pressure.

[0026] To fulfil the object, the present invention proposes a leaktesting apparatus for leak testing at least one closed and filledcontainer, whereby the content of the container comprises at least oneliquid component, which comprises at least one sealingly closable testcavity and at least one evacuation pump operationally connected to thetest cavity and further at least one pressure sensor operationallyconnected to the test cavity, whereby the evacuation pump is selected soas to be able to pump the test cavity to at least vapour pressure of theliquid component of the container content, approx. at room temperatureand the pressure sensor is a vacuum pressure sensor, preferablycomprising at least a Pirani sensor stage.

[0027] Preferred embodiments of the inventive method and inventiveapparatus are claimed in the dependent claims 2 to 44 and 45 to 63respectively. The inventive method and apparatus may preferably be usedas defined in the claims 64 and 65. Thereby, it must be pointed out thatbesides of leak testing of smaller containers, the present inventionmakes it possible to permanently monitor tightness of huge tank plants,as for gasoline, gases etc., e.g. on train or street transports, therebygenerating an alarm signal as soon as a leak is detected.

SHORT DESCRIPTION OF THE FIGURES

[0028] The present invention will now additionally be described with thehelp of figures showing specific and today preferred examples ofrealising the present invention. Such figures show:

[0029]FIG. 1: qualitatively the dependency of vapour pressure fromtemperature of a liquid;

[0030]FIG. 2: schematically an inventive test apparatus operatingaccording to the inventive method;

[0031]FIG. 3: qualitatively the time course of the pressure of thesurrounding of a container to be inventively tested for explaining theinventive method and apparatus operation;

[0032]FIG. 4: in a functional block diagram a preferred form ofrealisation of an inventively operated inventive test apparatus;

[0033]FIG. 5: as a functional block diagram a preferred form ofrealisation of the evaluating electronic at an inventive apparatusperforming the inventive method;

[0034]FIG. 6: schematically batch operation of an inventive apparatus;

[0035]FIG. 7: schematically a test cavity for testing flexible wallcontainers;

[0036]FIG. 8: in a perspective view one half of a test cavity forinventively testing three containers as a batch;

[0037]FIG. 9: schematically a double-wall tank directly used to performthe inventive method with an inventive apparatus so as to survey tankleakage;

[0038]FIG. 10: schematically a preferred sealing at a test cavity of theinventive apparatus;

[0039]FIG. 11a to 11 c: show the pressure courses on testing cycles,whereat the containers or medical application blisters are eitherlargely or even very largely leaking (FIG. 11a), or have only a smallleak (FIG. 11b), or are to be considered unleaky (FIG. 11c). The testsare performed with test cavities according to FIG. 8 without impedancemeasurement and thus without electrodes 32, 34.

[0040]FIG. 12 a signal flow/functional block diagram of the simplifiedpreferred embodiment of an evaluation unit for operating the inventivemethod at an inventive apparatus;

[0041]FIG. 13 in a pressure versus time diagram the statisticalvariation of pressure courses measured at unleaky containers or at testcavities void of any containers,

[0042]FIG. 14 in a simplified functional block/signal flow diagram apart of the inventive apparatus operating according to preferred mode ofthe inventive method, thereby forming a dynamic reference value for leaktesting by means of a subsequently updated averaging;

[0043]FIG. 15 in a simplified signal versus time diagram qualitativelythe preferred inventive method and accordingly operation of a preferredinventive apparatus, whereby dynamically updated reference values areformed for leak identification;

[0044]FIG. 16 a simplified signal flow/functional block diagram showinga further preferred mode of operation of the inventive method andrespectively of the inventive apparatus, wherein a dynamically updatedaverage signal is formed as the basis for a reference value to becompared with a pressure difference signal evaluated during containertesting;

[0045]FIG. 17 in arbitrary unit over the time axis pressure measurementsat subsequently operated test cavities of an inventive apparatus withmultiple cavities to show dynamic update of an average signal, whereonreference values for comparison are based, leading to leakageidentification;

[0046]FIG. 18 in a simplified schematic representation, a test cavityaccording to the present invention, which is pivoted during testing;

[0047]FIG. 19 the effect of pivoting the test cavity according to FIG.18 on the relative location of a leak with respect to filling product;

[0048]FIG. 20 in a simplified functional diagram provision of acalibration standard leak to calibrate the inventive apparatus asperforming the inventive method.

[0049] In FIG. 1 there is qualitatively shown the course of vapourpressure p_(V)(T) in the pressure versus temperature diagram. At apredetermined temperature T_(x) a liquid starts to evaporate when therespective vapour pressure p_(Vx) is reached. Above the vapour pressurecourse the material is liquid, below the material is gaseous.

[0050] According to FIG. 2 an inventive apparatus comprises a testcavity 1 with a sealingly closable cover 3. A vacuum pump 5 connected tothe test cavity 1 which may be a drag pump or a rotational piston valvepump or a diffusion pump or a turbo vacuum pump as a turbo molecularpump. This depends on the degree of vacuum which shall be establishedwithin cavity 1. Further, there is provided a vacuum pressure sensor 7as e.g. a Pirani sensor, which measures the pressure prevailing in thetest cavity 1. At least one closed container 9, which is filled at leastto some extent with a filling product containing at least one liquidcomponent is introduced through opened cover 3 into the test cavity 1which is then sealingly closed. By starting operation of vacuum pump 5the surrounding of container 9 and thus the intermediate volume V oftest cavity and container 9 is lowered.

[0051] According to FIG. 3 starting at ambient pressure p_(o) thepressure in volume V is lowered at least down to the value p_(V) whichaccords to the vapour pressure of the liquid component within thefilling good of the container 9. It is advisable to select a vacuum pump5 which enables to evacuate the test cavity 1 down to a pressure whichis at least one, preferably two and even more preferred three decadeslower than the vapour pressure p_(V) of the liquid content of thefilling product.

[0052] The test is preferably performed at room temperature, i.e. at atemperature T of about 20° C. If the liquid content is water then thevapour pressure p_(V) of water at room temperature is about 20 mbar andit then is preferred to provide an evacuation pump 5 which is able toevacuate the test cavity to about 10⁻² mbar.

[0053] If the container provided in the test cavity 1 having arelatively rigid wall 11 is not leaky, then qualitatively the pressurein volume V will follow the course (a) according to FIG. 3 down to themore or less constant value of pressure, which may be reached by thattype of vacuum pump installed. If, on the other hand, the container 9 isleaky as schematically shown in FIG. 2 e.g. at location 13, then a smallamount 14 of liquid component of the filling good will be drawn throughthe leak 13 out of the container 9 and as soon as the pressureprevailing in the volume V becomes p_(V) starts to evaporate into thevolume V. As qualitatively shown in FIG. 3 this leads to a pressureversus time course according to (b), i.e. evaporation of the liquidleads to a pressure rise in volume V, counteracting the action of thevacuum pump 5. The vacuum pump 5 will have to remove additionally thevapour to finally achieve a vacuum level according to course (a). If theleak is situated at an area of the container 9 where air is entrapped,as in FIG. 2 at 13′, then evacuation of volume V will first lead tosuctioning air out of the container, again counteracting the operationof vacuum pump 5, then the liquid content within container 9 will startto evaporate within the container and vapour will be suctioned out ofleak 13′. This, too, will lead to a pressure rise in volume V,counteracting the pressure course which would be followed if just airhad to be removed by vacuum pump 5.

[0054] By means of the vacuum sensor 7 the course of pressure in thevolume V is monitored. Experiments have shown that largely independentof the amount of volume V in a test cavity a significant difference ofpressure according to the courses (a) and (b) of FIG. 3 is reached aftera time span τ of a few seconds (one to three seconds) and at a leaksmaller than 1 micron (0.02 μm), the pressure difference between a leakyand an unleaky container being of about one pressure decade.Measurements were performed with water as liquid content.

[0055] Although it is absolutely possible to measure the absolutepressure in volume V, e.g. after the time span τ to detect leakage ofthe container a pressure difference measurement is preferred, as willfirst be explained with the help of FIG. 4. Back to FIG. 2 the pressuresensor 7 is operationally connected to an evaluating unit 15, whereatespecially leak indicative threshold values are preset, as schematicallyshown by means of a presetting unit 17. The output of the evaluationunit 15 is a two-state signal indicating leaky or not leaky.

[0056] According to FIG. 4 the output of the vacuum sensor 7 is input toa storage unit 19, controlled by a timing control signal s₁, asschematically shown via switch S. According to FIG. 3 this is performedat a first point in time t₁. At a second point in time, according toFIG. 3 t₂, the output of the storage unit 19 and the output of thesensor 7 are connected to respective inputs of a difference forming unit21, which generates an output signal which accords with the pressuredifference Δ p of FIG. 3.

[0057] A further, most preferred realisation of the evaluationelectronic is shown in FIG. 5. The output signal of sensor 7 is input toa conversion unit 121, which comprises, as an input stage, an analogueto digital converter 121 a, followed by a digital to analogue converter121 b. The output of the converter stage 121 is fed to a differenceamplifier unit 123, which additionally receives directly the outputsignal from sensor 7. The output of the difference amplifier unit 123,according to the difference unit 21 of FIG. 4, acts on a furtheramplifier unit 125, the output of which being superimposed at 128 to itsinput via storage unit 127. The input of the storage unit 127 is fedfrom the output of unit 125. A timer unit 129 time controls thearrangement. For storing a first pressure value from sensor 7, accordingto FIG. 3 at time t1, the timer unit 129 enables a conversion cycle atunit 121, so that a reconverted analogue output signal el_(o) appears atthe output. Simultaneously, the substantially same signal from sensor 7is applied as signal el to the second input of unit 123. Thus, at theoutput of unit 125, a zero signal should appear. Nevertheless, ingeneral a zero-offset signal will appear at the output of unit 125,which signal is stored in the storing unit 127, enabled by the timingunit 129. At time t2 no conversion is triggered at the unit 121, so thatthere appears at the input of amplifier 123 directly from sensor 7 thepressure value prevailing at t₂ and, from stage 121, the stored pressurevalue which was prevailing at t₁. Further, the zero offset signal whichwas stored in unit 127 is superimposed as a offset-compensating signalso that the resulting signal at the output of amplifier unit 125 iszero-offset compensated.

[0058] This allows a very accurate measurement of pressure difference Δpaccording to FIG. 3.

[0059] If the container under test has a large leak, then, and accordingto FIG. 3 course (c) the pressure prevailing in the volume V of the testcavity 1 will have just from the beginning of operating the vacuum pump5 a different course. This may easily be detected, e.g. by comparing ata previous point in time t₀ the output signal of sensor 7 with apredetermined threshold value (not shown), and if such threshold valueis not reached by the actual pressure, the effect of the vacuum pump 5on test cavity 1 is disabled. This to avoid that, with a larger leak, ahuge amount of content of the container is suctioned into the testcavity and contaminates that cavity.

[0060] As was mentioned, the proposed method accurately functionslargely independently from the volume V between test cavity 1 and the atleast one container to be tested. This allows, according to FIG. 6, tosimultaneously test batches 9′ of containers 9, thereby maintainingaccuracy of detecting whether one or more than one of the containers 9leak. Further, the fact that detection accuracy is not critical withrespect to difference volume V leads to the possibility of providing onetest cavity 1 for a multitude of differently shaped and different-volumecontainers 9 to be tested therein.

[0061] If the wall of a container to be tested may not mechanicallywithstand the pressure loading of approx. 1 bar, then, and asschematically shown in FIG. 7, a test cavity 1′ with cover 3′ isprovided which snugly fits with the shape of the container 9. Thereby,protrusions 20, as schematically shown in FIG. 7, prevent that by effectof the evacuation the walls of the container are firmly suctioned on tothe inner wall of the test cavity and thus make sure that there remainsa volume V between container and test cavity wall for being evacuatedaccording to the invention. Such protrusions 20 may be realised by amesh or grid inlay or, and preferably, by mechanically roughening theinner wall of the cavity, so that micro-embossments sustain the wall ofthe container, thereby leaving a continuous interspace as volume V.

[0062] As shown in dashed line in FIG. 7 it might further beadvantageous, e.g. when closing the cover 3 or 3′ of the cavity, tomechanically bias a part of the container's wall inwardly, therebyincreasing the inner pressure of the container 9 and additionallypressing liquid component of the filling product out of a leak if such aleak is existent.

[0063] According to FIG. 9 the method and apparatus according to thepresent invention may be used to monitor huge tanks with respect toleakage. In FIG. 9 there is shown a tank with doublewall, namely with aninner wall 23 and an outer wall 25. Testing tightness of both thesewalls is performed by using the intermediate volume of the two walls, asvolume V according to FIG. 2. Such a technique may be applied e.g. fortanks on road or rail vehicles or for huge stationary tank plants, e.g.for gasoline.

[0064] In FIG. 8 there is shown one half 1 a of a test cavity 1 forapplying the inventive method in an inventive apparatus on threecontainers at 29 as on small plastic containers for medical appliance.The containers may have flexible walls as the test cavity 1 snugly fitstheir shape. There is further shown another technique to rapidly detectwhether one of the containers has a large leak. There are providedimpedance measurement electrodes 32 and 34 integrated in the wall of thecavity 1 and mutually electrically isolated. They are connected to animpedance or, preferably, resistance measuring unit 35. If by applying avacuum to the test cavity, preferably with a roughened interior wall,liquid filling content is suctioned to the outside of the containerwall, this is quickly detected by an abrupt change of impedance measuredbetween the electrodes 32 and 34. The output of the impedance measuringunit 35 disables (not shown) further evacuation of the test cavity 1.

[0065] Once a test cavity has been spoiled by outpouring filling good ofa leaking container it is cleaned, either by cleaning evacuation and/orpouring with a gas, preferably with nitrogen, and/or by heating. In FIG.8 there is shown a feeding line for a flushing or cleaning gas,controllably fed from a gas tank 37 to a contaminated test cavity 1,which gas preferably is nitrogen.

[0066] Two cavity halves, 1 a according to FIG. 8 are sealingly put oneupon the other to complete a test cavity 1 according to FIG. 2.

[0067] If in-line testing of containers shall be performed, for whichthe present invention is especially suited due to its short measuringcycle, more than one, namely a set of several test cavities is provided,e.g. on a carousel, which are automatically loaded with containers to betested (not shown) from a conveyor and which perform simultaneously thedescribed testing technique. If one of the containers tested in suchcavity is detected to be leaky, then the respective cavity is notreloaded with a further container afterwards, but this cavity ismaintained empty during the measuring cycle on a next set of containers.Meanwhile, the cavity kept unloaded is cleaned, as was described, eitherby evacuation and/or gas flushing and/or heating.

[0068] Obviously, there must be realised a good vacuum-tight sealingbetween a cover 3 or 3′ of the test cavity and the main body of the testcavity 1 or between the two halves 1 a of test cavity according to FIG.8. This is realised preferably by providing at least a pair of parallelseals 28 as of concentric O seals and by separately pumping anintermediate space 29 between such seals, as shown in FIG. 10. If thecontainer to be tested contains a filling product with more than onespecific liquid component, the vapour pressure of that component isselected for leak detection which has the highest vapour pressure, i.e.which component starts to evaporate at relatively highest pressure.Thereby, viscosity has to be considered too, i.e. a component is to beselected for defining the vapour pressure, which component is liquidenough to penetrate smallest leaks. By evacuating the test cavity downto a pressure which is significantly lower than the vapour pressure ofany liquid component it becomes uncritical which vapour pressure valveis to be considered.

[0069] Pressure versus time courses as measured according to theinventive method and with an inventive apparatus, both in preferredmode, are shown for containers with large leaks in FIG. 11a, for smallleaks in FIG. 11b and for unleaky containers in FIG. 11c.

[0070] These figures shall be discussed in connection with FIG. 12,which shows a preferred monitoring and control unit according to units15, 17 of FIG. 2.

[0071] According to FIG. 11a the timing unit 201 of FIG. 12 initiates attime t₁₀ evacuation of a test cavity 103 by means of the pumpingarrangement 105. This is shown in FIG. 12 by the evacuation start signalEVST/t₁₀.

[0072] After a fixed predetermined amount of time ΔT of e.g. 0.75 sec.the output signal of the pressure sensor within test cavity 103 (notshown in FIG. 12), A₅, becomes compared with a first reference signalpreset at a presetting source 107, RFVGL. To this target, comparatorunit 109 is enabled by timer unit 201 at t₁₀+ΔT.

[0073] If after time span ΔT the actual monitored pressure according toelectric signal A₅ of FIG. 12 has not reached the value of RFVGLaccording to course I of FIG. 11a, this means that a very large leak VGLis present. This is detected at comparator 109 generating the outputsignal A₁₀₉. If according to the characteristics shown in the block 109of FIG. 12 the output signal of this comparator unit 109 enabled att₁₁=t₁₀+ΔT is e.g. still at a high level indicating presence of a VGL,this is output at the VGL output. If the pressure prevailing in thesurrounding of the container 103 under test, i.e. in the test cavity,has reached and crossed reference level RFVGL according to course II ofFIG. 11a, the VGL output signal is not generated.

[0074] As will be explained later, occurrence of the VGL signalpreferably stops the evacuation cycle because contamination of thevacuum pump 105 may have occurred or might occur due to the very largeleak of the container under test.

[0075] As shown by the course II of FIG. 11a as VGL does not occurevacuation continues up to a further moment of time t₁₃. At the time t₁₃the timer unit 201 disables pumping arrangement 105 and disconnects asby a valve 106 the pumping arrangement from chamber 103. Further, timerunit 201 enables comparator unit 111, to which a further reference valueRFGL is led, generated by a reference signal source 113. If at t₁₃ thepressure prevailing in the surrounding of the test cavity has notreached RFGL then comparator unit 111 generates an output signal GLindicating that the container under test has a large leak. Here again,and as will be further explained later on, some reactions are taken withrespect to further operation of the testing system.

[0076] If either the signals VGL or GL are initiated by the respectivecomparators 109, 111, the timer unit 201 is principally reset becausethe testing has been completed and the quality of the instantaneouslytested container established has been identified. This is schematicallyshown if FIG. 12 by the signal RS₂₀₁. If not reset, shortly after t₁₃the value A₅ (t₁₃) of the pressure prevailing in the surrounding of thecontainer is stored in a holding or storing unit 117. The output of theholding or storing unit 117 is led to one input of a difference formingunit 119, whereas the second input of this unit 119 is connected to theoutput A₅ of the pressure sensor monitoring the pressure in thesurrounding of the container under test. After a presettable test cycletime T_(T) starting at t₁₃, as schematically shown by unit 121 of FIG.12, the pressure difference DP at the output of unit 119 is evaluated,as represented in FIG. 12 by switching unit 123. This pressuredifference DP is fed to a further comparator unit 125 enabled at thelapse of testing time T_(T). By means of a further reference valuesource 127 the reference value DPREF is fed to the comparator unit 125.As will be explained later, the value of DPREF may controllably bevaried in time and/or a reference value φ_(R) to which DPREF is referredto may also controllably be varied in time.

[0077] If DP at time t₁₃+T_(T) is larger than the reference value DPREF,then a signal FL is generated at unit 125, indicating presence of a fineleak FL in the container under test. This according to the situation asshown in FIG. 11b. If DP does not reach DPREF, then the container isconsidered unleaky as none of the signals VGL, GL and FL have beengenerated. This according to FIG. 11c.

[0078] If the VGL signal is generated according to FIG. 12, theevacuation pump 105 is immediately disconnected from any testing chamber103 it is connected to, be it a single chamber or be it in an in-lineprocessing where one pump 105 is parallel connected to a multitude oftesting chambers 103, from all such chambers. This because at a verylarge leak the vacuum pump 105 could have been contaminated by leakingcontent of the container. It thereby is absolutely possible to providefor such a case a redundant pumping arrangement which may be connectedto the one or the more than one testing chambers to continue testing,whereas the possibly contaminated first pumping arrangement isreconditioned.

[0079] In a multiple chamber in-line testing system, as e.g. in acarousel testing plant with a multitude of testing chambers, occurrenceof the signal GL indicating a large leak and possibly also theoccurrence of the signal FL indicating for a fine leak leads preferablyto disabling or “bypassing” that chamber with the leaky container fromfurther being supplied with containers to be tested, whereas the otherchambers are still operating and performing tests on newly suppliedcontainers. This bypass of a testing chamber, whereat a container hasbeen identified as heavily or even slightly leaking, is performed so asnot to influence further testing results at that chamber which wouldn'tthus be representative anymore due to content of the leaky containerhaving possibly contaminated that chamber.

[0080] This bypassed chamber is reconditioned during further testingcycles at the other chambers.

[0081] Reconditioning may be done by heating that chamber, flushing itby a liquid and/or a gas, especially by a heater gas. Whether or notthat chamber has been properly reconditioned is checked by having ittested as if it was filled with a container to be tested. Thereby, thecondition of proper reconditioning is indicated if DP according to FIG.12 at that empty chamber does e.g. not reach DPREF or an appropriatelyset “Empty Chamber DP-REF”-value (ECDP-REF).

[0082] Such ECDP-REV may be provided by measuring DP_(e) at the clean,empty test chambers and by storing these measuring values DP_(e) asrespective reference values for testing the chambers on properreconditioning.

[0083] When looking to the FIG. 11a to 11 b, it may by recognised thatsetting the reference value RFGL and especially setting of the referencepressure difference value DPREF may be very critical and may largelyinfluence accuracy of the system. Thereby, influences as surroundingtemperature, moisture of ambient air, slight contamination of pump etc.,may influence the prevailing pressure course and lead to false resultsif these critical reference levels and especially DPREF are set forutmost accuracy.

[0084] In FIG. 13 there is quantitatively shown the pressure courseaccording to the courses of FIG. 11a to 11 c, but measured at testcavities void of containers. At t₁₃ there occur statisticallydistributed slightly different pressure values. Thus, before beginningtesting of containers at a multiple test cavity plant, the unfilled,tightly closed test cavities are tested according to FIG. 13 toestablish an average (RFGL)_(m). The value of RFGL as used at thecomparator 111 of FIG. 12 or as used according to the FIGS. 11a to 11 cis found in that an offset value ΔRFGL is added to (RFGL)_(m). It mustbe pointed out that ambient parameters as temperature, humidity ofambient air etc. may be considered constant during the calibrating cycleperformed at the empty and conditioned test cavities and leading to themeasuring results according to FIG. 13. Nevertheless, during ongoingtime as during on-line testing, these disturbing parameters may slowlychange and may vary (RFGL)_(m).

[0085] Every time during multiple or in-line testing, be it subsequentlywith a single test cavity or consecutively with a multitude or at leastmore than one test cavity, at the respective time t₁₃, up to which therespective container has been identified as not heavily leaky, theactual output signal of the pressure sensor is entered into an averagingunit 113, wherein the last m values of actual pressure of not heavilyleaky containers are averaged. The output average result signal accordswith (RFGL)_(m) of FIG. 13, but varies in time, e.g. due to varyingambient parameters. To the output average result {overscore (A5)} andaccording to FIG. 13 the offset ΔRFGL is added, the result of thataddition is a dynamically varying reference value RFGL, which is appliedto comparator unit 111 of FIG. 12. This dynamically varying referencevalue RFGL is shown in FIG. 15, starting from an initial setting, ase.g. found as was explained with the help measurements at empty testcavities 103.

[0086] As may clearly be seen now from FIG. 15, the average pressurevalue {overscore (A5)} (t₁₃) is now the basis for also referring DPREFto. Therefore, and as shown in FIG. 12, the difference pressurereference value DPREF is not referred to an absolute static value asφ_(R), but is referred to {overscore (A5)}.

[0087] An even further improvement of accuracy is reached as will now bedescribed, which may be realised separately or additionally to realisinga dynamic RFGL and based thereon a dynamic upper limit of DPREF. Therebyand according to FIG. 16 at the end of the time span TT the actualpressure difference DP is led to an averaging unit 135 whenever theoutput signal FL indicates that the container under test is unleaky. Theoutput signal of unit 135 which accords to an average pressuredifference signal {overscore (DP)} averaged over the last m test cycles,is offset by an amount ΔDP, the result thereof being used as DPREFsignal applied at unit 127 of FIG. 12.

[0088] Looking back on FIG. 15, whereat, as discussed before, a constantDPREF signal was applied the technique of averaging DP results, asschematically shown with a course (DPREF)_(t), in a dynamically varyingcheck value DPREF, varying according to variations of disturbingparameters, influencing such pressure difference.

[0089] it is clear that provision of a dynamically varying (DPREF)_(T)signal according to that representation in FIG. 15 could be realisedwithout providing a dynamically varying base value {overscore (A5)}, inreferring (DPREF)_(t) to a stable, constant value φ_(R), as shown inFIG. 12 in dashed representation instead of referring to a dynamicallyvarying {overscore (A5)} value.

[0090] It is evident that preferably the evaluations of the outputsignal A₅ of the one or more than one test cavities is performeddigitally, i.e. after analogue to digital conversion of the outputsignal of the respective sensor or sensors.

[0091] In FIG. 17 there is shown over the time axis and in arbitraryunits the actual pressure difference values DP measured successively ata multitude of test cavities of an in-line testing plant. According toFIG. 16 the calculated average pressure difference {overscore (DP)} isshown and the offset ΔDP finally leading to (DPREF)_(t) according toFIG. 15 or 16. As may clearly be seen, the average {overscore (DP)} andthus (DPREF)_(t) vary in time and along successive testing, wherebypressure difference values as at A, which are higher than theinstantaneously prevailing (DPREF)_(t), are disregarded with respect toinfluencing the averaged {overscore (DP)}, as such measurements are dueto leaky containers according to FIG. 11b.

[0092] Further, whenever the test of a container within a specific testcavity results in a leak-indication for a predetermined number ofsubsequent tests, as e.g. three times subsequently, such test cavity isalso bypassed for further testing and is considered as contaminated oras leaky itself, thus being reconditioned. Such a test cavity is likelyto have been contaminated during succeeding testings at leaky containersor is likely not to be tight, which will be recognised duringreconditioning and testing on proper reconditioning too, as wasdescribed above.

[0093] Further, and as was already mentioned, for some containers to isbe tested and especially for some filling products it is advisable toheat the test cavities to a predetermined temperature which ispreferably controlled at each test cavity, e.g. by a negative feedbacktemperature control. Thereby, the temperature-dependent evaporationpressure of the filling product is set within a predetermined pressurerange. Such heating is thereby preferably accomplished in a pre-heatingcycle before the actual testing cycle according the figures 11 a to 11 cis performed.

[0094] As was mentioned above, a leak in a container will be identifiedirrespective of the fact whether such leak is in an area of container'swall exposed to entrapped air within the container or to the fillingproduct. Nevertheless, for some filling goods as e.g. with particulatecontent in liquid, there might occur differences with respect to time arespective pressure difference develops in the surrounding of thecontainer under test.

[0095] Therefore, and as schematically shown in FIG. 18, it may beadvisable in some cases to provide the one or the several test cavities103 for the container to be tested 9 to be movable. This is e.g.accomplished by mounting the test cavities 103 pivotable with respect toa pivot axis A and driven via a rotational axis 140. Thereby, leads toand from the pressure sensor within such test cavity, to and from aheating arrangement at such a test cavity etc. may be led through thedriving axis 140. The cavity 1, 103 is preferably not rotated, but isrotatably oscillated as shown by ±φ in FIG. 18. By this technique, andas schematically shown in FIG. 19, a leak L is moved into air and intoliquid contact, so that testing will consider vaporising of liquidcontent whenever it occurs, be it in the position according to FIG. 19aor in the position according to FIG. 19b.

[0096] Proper functioning of the testing apparatus and calibration ofthe evaluation unit, be it a one-chamber tester or a at multiple-chambertesting plant as for in-line testing, is further preferably accomplishedwith the help of a standard leakage arrangement which is preferablymounted on the test plant, so that recalibration and/or overall testingof the plant may be accomplished whenever desired. The arrangement ofsuch a standard or calibration leak arrangement is shown in FIG. 20.

[0097] According to FIG. 20 there is provided in the line from a testcavity, as 103 according to FIG. 12, to the vacuum pump 105 a needlevalve 142, which is adjustable but which preferably is preset notvariable by the user of the plant on a predetermined leakage value. Viathe needle valve 142, the line to the vacuum pump 105 is connected to aliquid reservoir 144, which preferably is filled with distilled water.Via a pressurising line and valve 146 the reservoir 144 may beadjustably pressurised. The needle valve is set to such a value that nodistilled water of reservoir 144 will penetrate into the connection lineof chamber 103 to vacuum pump 105, but only vapour. Nevertheless, byadjusting pressurisation of the water within reservoir 144 via line andvalve 146 a leak of different and varying extent may be simulatedwithout liquid penetrating and spoiling chamber and/or connection lineand/or vacuum pump. For a plant with a multitude of testing cavitiessuch a calibration arrangement with needle valve 142 may centrally beprovided and connected in parallel to all chambers 103, as in such aplant preferably there is provided one central pumping arrangement 105acting in parallel on all the chambers or cavities provided.Alternatively such a calibration arrangement may be provided separatelyfor each of the chambers 103 provided.

[0098] It has been recognised that by applying the described techniqueof leak testing by lowering the surrounding pressure of a containerunder test below vapour pressure of a liquid component of its content,it is mostly not necessary to additionally provide resistancemeasurements, as was explained with the help of FIG. 8, so that, at therespective test chambers, the electrode arrangements and measurementunits may be omitted, which significantly reduces costs for the overallplant and its complexity. The invention is especially suited for testingvials or blisters, especially for medical appliances, in-line with theirproduction by checking every singly vial or blister. If and asschematically shown in FIG. 6 a multitude of containers 9 aremechanically linked together to form a set of such of containers,clearly such a set is considered as one container with respect to leaktesting.

[0099] With the inventive method and apparatus as for blisters theentire testing cycle, i.e. from t₁₀ to the end of T_(T) according to theFIGS. 11 is performed in less than 2 sec. This leads at an in-line plantwith a multitude of test cavities, e.g. with 24, e.g. arranged on acarousel, to a very high throughput.

1. A method for leak testing at least one closed and filled container,whereby the content of said container comprises at least one liquidcomponent and wherein a pressure difference is applied across at least apart of the wall of the container, which part is to be tested, saidpressure difference being directed towards the surrounding of saidcontainer and wherein the pressure in said surrounding is monitored as aleak indicative signal, characterised by the fact that said pressuredifference is established by lowering said pressure in said surroundingat least to a value according to vapour pressure of said at least oneliquid component.
 2. The method of claim 1 , characterised by the factthat said pressure in said surrounding is lowered towards a pressurevalue which is lower than said vapour pressure by at least two,preferably by at least three decades.
 3. The method of claim 1 , whereinmore than one liquid component is present, characterised by the factthat said vapour pressure is the higher vapour pressure of the vapourpressures of said at least two components.
 4. The method of claim 1 ,characterised by the fact that said test is performed at roomtemperature.
 5. The method of claim 1 , characterised by the fact thatsaid pressure monitored as said leak indicative signal is monitoredafter reaching said vapour pressure.
 6. The method of claim 1 ,characterised by the fact that said pressure monitored is sampled at afirst point in time, resulting in a first pressure measuring signal andis sampled at a second, subsequent point in time, resulting in a secondpressure measuring signal and that a pressure difference formed by saidtwo pressure measuring signals is evaluated as leak indicative signal.7. The method of claim 6 , characterised by the step of generating saidfirst and second measuring signals as electrical signals, and storingsaid first signal at least up to said second point in time.
 8. Themethod of claim 6 , characterised by the step of providing a pressuremeasuring sensor in said surrounding and operationally connecting saidsensor to both inputs of a difference forming unit at said first pointin time, generating a zero offset signal dependent from the outputsignal of said difference forming unit, storing said zero offset signaland compensating zero-offset at said signal difference of said twomeasuring signals by said stored zero offset signal.
 9. The method ofclaim 6 , characterised by the step of providing a pressure measuringsensor in said surrounding and comparing the output signal of saidsensor with one or more than one predetermined signal values.
 10. Themethod of claims 6, characterised by the step of storing said firstmeasuring signal by means of an analogue to digital converter, enablefor conversion at said first point in time.
 11. The method of claim 10 ,characterised by the step of reconverting the digital output signal ofsaid analogue to digital converter into an analogue signal.
 12. Themethod of claim 1 , further comprising the step of simultaneouslytesting a batch of said containers as one container.
 13. The method ofclaim 1 , further comprising the step of performing an impedancemeasurement at or at least adjacent to said part of said wall in saidsurrounding, preferably a resistance measurement with DC and enabling ordisabling further lowering of said pressure in said surrounding by theresult of said impedance measurement.
 14. The method of claim 1 ,characterised by the step of providing a test cavity with a test chambersnugly fitting the outer shape of said at least one container, therebymaintaining at least at said part a residual volume to be lowered inpressure and between said part and the wall of said test cavity.
 15. Themethod of claim 1 , characterised by the step of providing a test cavityfor said at least one container, said test cavity defining for a testchamber significantly larger than the volume of said container.
 16. Themethod of claim 1 , characterised by the step of providing a test cavityfor said container and cleaning at least said test cavity after acontainer therein has been detected as leaking, said cleaning beingperformed by evacuating said cavity and/or by flushing with a gas,preferably by nitrogen and/or by heating.
 17. The method of claim 1 ,characterised by the step of inline testing a series of said containersin a set of test cavities and further comprising the step of disablingtesting in a test cavity for at least one testing cycle if the containerpreviously tested therein has turned out to be leaky.
 18. The method ofclaim 1 , further comprising the step of increasing internal pressure ofsaid at least one container by mechanically biasing at least a part ofits wall inwardly.
 19. The method of claim 1 , for at least onecontainer, wherein said one component is water, characterised by thestep of evacuating said surrounding to less than 20 mbar, preferably toapprox. 10⁻² mbar.
 20. The method of claim 1 , further comprising thesteps of initiating lowering said pressure with a predeterminedsuctioning power; identifying for a large leak if said pressuremonitored does not reach a first predetermined pressure value in apredetermined time; disabling further lowering of said pressure;monitoring a change of said pressure monitored during a furtherpredetermined time and identifying a small leak or no leak dependent onthe extent of said change.
 21. The method of claim 1 , furthercomprising the step of lowering said pressure during a predeterminedtime and at a predetermined suctioning power.
 22. The method of claim 1, further comprising the step of establishing a maximum threshold valuefor said pressure monitored to be reached after a predetermined time oflowering and disabling a pumping arrangement from lowering said pressureof said surrounding if said pressure monitored does not reach saidmaximum threshold value at said predetermined time.
 23. The method ofclaim 22 , said disabling comprising disconnecting said pumpingarrangement from said surrounding.
 24. The method of claim 23 , furthercomprising the step of switching said surrounding to a further pumpingarrangement for performing a subsequent leak testing cycle.
 25. Themethod of claim 1 , further comprising the steps of providing saidsurrounding within a test cavity for said at least one container anddisabling said test cavity for at least one further testing cycle if aleaky container is detected in said test cavity.
 26. The method of claim25 , further comprising the step of reconditioning said test cavityduring said at least one testing cycle.
 27. The method of claim 26 ,further comprising the step of performing said reconditioning by atleast one of heating, gas purging and liquid purging.
 28. The method ofclaim 26 , further comprising the step of checking whether said testcavity is properly reconditioned by performing said leak testing at saidtest cavity empty from a container to be tested.
 29. The method of claim1 , further comprising the steps of comparing a signal derived from saidpressure monitored with at least one threshold value to identify leakagecondition of said container and deriving said at least one thresholdvalue from said pressure monitored at a test cavity defining for saidsurrounding and void of such container.
 30. The method of claims 1,further comprising the step of monitoring said pressure at at least onepredetermined moment after starting said lowering, comparing a signalderived from said pressure monitored at said predetermined moment with athreshold value for identifying leak-condition of said container,enabling a further signal derived from said pressure monitored to beaveraged with such further signals for containers previously tested ifsaid identifying reveals an unleaky container and deriving saidthreshold value from the result of said averaging.
 31. The method ofclaim 30 , wherein said signal derived from said pressure monitored atsaid predetermined moment is a difference signal to a signal derivedfrom said pressure monitored at a further predetermined moment.
 32. Themethod of claim 1 , further comprising the step of providing at leastone test cavity for said container and calibrating said pressuremonitored by performing said leak testing at said test cavity void ofcontainer and connected to a reference leak arrangement.
 33. The methodof claim 32 , further comprising the step of providing said referenceleak arrangement by a needle valve to a reservoir containing a liquidand being controllably pressurisable.
 34. The method of claim 33 , saidreservoir containing distilled water.
 35. The method of claim 33 ,further comprising the step of controlling said reference leak andpressure to prevent liquid to leak therefrom and to enable vapour ofsaid liquid to transit therethrough.
 36. The method of claim 1 , furthercomprising the step of providing said surrounding in a test cavity andsubsequently performing said leak testing with said test cavity ondifferent of said containers, thereby disabling said test cavity fromfurther testing if a leaky container was identified therein for apredetermined number of subsequent tests.
 37. The method of claim 1 ,further comprising the step of heating said surrounding during saidtesting to a predetermined temperature.
 38. The method of claim 1 ,further comprising the step of performing a testing operation foridentifying larger leaks of said at least one container, previously toperforming said method for leak testing.
 39. The method of claim 1 ,further comprising the step of inline testing a series of saidcontainers in a set of test cavities, thereby converting said pressuremonitored in each of said test cavities in an electrical signal andgenerating at least one reference electrical signal by lowering saidpressure in said test cavities void of containers.
 40. The method ofclaim 1 , comprising the steps of providing a set of test cavities eachdefining for one of said surroundings and monitoring said pressure insaid surroundings at at least one predetermined moment after startinglowering said pressure in said respective surroundings, comparing,respectively, a signal derived from said pressure monitored at saidrespective predetermined moment with a common threshold value foridentifying leak condition of containers in said test cavities, enablinga further signal derived from said pressure respectively monitored to beaveraged with such further signals generated previously if saididentifying reveals an unlaky container and deriving said commonthreshold value from the result of said averaging.
 41. The method ofclaim 40 , wherein said signal respectively derived from said pressuremonitored at said predetermined moment is a difference signal withrespect to a signal derived respectively from said pressure monitored ata further predetermined moment.
 42. The method of one of claims 17, 25,36 further comprising the step of reconditioning said test cavitydisabled from testing and re-enabling said test cavity for testing afterreconditioning.
 43. The method of claim 42 , further comprising the stepof performing said reconditioning by at least one of heating, gasflushing, liquid flushing.
 44. The method of claim 42 , furthercomprising the step of checking whether said test cavity is properlyreconditioned by performing said leak testing at said test cavity voidof said at least one container.
 45. A leak testing apparatus for leaktesting at least one closed and filled container, whereby the content ofthe container comprises at least one liquid component and comprising: atleast one sealingly closable test cavity at least one evacuation pumpoperationally connected to said test cavity at least one pressure sensoroperationally connected to said test cavity characterised by the factsthat said evacuation pump is selected so as to pump said test cavitydown to at least vapour pressure of said component approx. at roomtemperature said pressure sensor is a vacuum pressure sensor.
 46. Theapparatus of claim 45 , characterised by the fact that said evacuationpump is at least one of a drag vacuum pump, a piston valve vacuum pump,a diffusion pump, a turbo vacuum pump.
 47. The apparatus according toclaim 45 , wherein said sensor comprises a Pirani sensor.
 48. Theapparatus of claim 45 , characterised by comprising a timing unitoperationally connecting the output of said sensor to an output of saidapparatus generating a leak indicative signal, once pressure within saidtest cavity has reached at least said vapour pressure value.
 49. Theapparatus according to claim 45 , said evacuation pump being selected soas to be able to pump said test cavity down to a pressure at leastsmaller than said vapour pressure by one, preferably by two decades oreven more preferred by three decades.
 50. The apparatus of claim 45 ,characterised by comprising a storage unit operationally connected tothe output of said sensor and a difference forming unit, one inputthereof being operationally connected to the output of said storageunit, the second input thereof being operationally connected to theoutput of said sensor, a timing unit operationally connecting the outputof said sensor at a first point in time to the input of said storageunit and operationally connecting at a second point in time the outputof said storage unit to said one input of said difference forming unitand the output of said sensor to said other input of said differenceforming unit.
 51. The apparatus of claim 50 , characterised by saidtiming unit enabling said operational connection of said output of saidsensor to said storage unit after said sensor has detected a pressurewithin said test cavity reaching said vapour pressure.
 52. The apparatusof claim 50 , characterised by said storage unit comprising an analogueto digital converter, said timing unit being operationally connected tothe conversion control input of said analogue to digital converter. 53.The apparatus of claim 45 , characterised by said timing unitoperationally connecting at said first point in time the output of saidsensor to both inputs of said difference forming unit and further bycomprising a further storage unit operationally connected to the outputof said difference forming unit and being enabled at said first point intime, the output of said further storage unit being operationallyconnected with the output of said difference forming unit at said secondpoint in time.
 54. The apparatus of claim 52 , characterised by adigital to analogue converter operationally connected to the output ofsaid analogue to digital converter, the output of said digital toanalogue converter being operationally connected to said differenceforming unit.
 55. The apparatus of claim 45 , wherein said test cavityhas a shape snugly fitting said at least one container and comprisingsustaining means at its inner wall to maintain a free space between thewall of said container and the wall of said test cavity once the volumetherebetween is lowered.
 56. The apparatus of claim 55 , characterisedby at least one pair of impedance measuring electrodes within saidcavity, connected to an impedance measuring unit, preferably to aresistance measuring unit, the output thereof switchingly enabling anddisabling further evacuation of said test cavity by means of saidevacuation pump.
 57. The apparatus of claim 45 , characterised by saidtest cavity being large enough to receive at least two, preferably amulti-container batch of said containers.
 58. The apparatus of claim 45, characterised by said test cavity being significantly larger than saidcontainer and thus being apt to flexibly receive differently shaped anddifferent volume containers.
 59. The apparatus of claim 45 ,characterised by said cavity comprising a removable cover and at least apair of seals around the opening opened by said cover, the space betweensaid two seals being pumped.
 60. The apparatus of claim 45 for in-linetesting a multitude of containers, characterised by a set of said testcavities to which a respective number of containers is fed for testing,further comprising control means to prevent a test cavity being filledwith at least one container to be tested once the container previouslytested in said one test cavity has been detected as leaking.
 61. Theapparatus of claim 45 , further comprising at least one cleaning gasline abutting into said cavity and being connected to a cleaning gastank, preferably containing nitrogen.
 62. The apparatus according toclaim 45 , comprising a multitude of test cavities, arranged on acarrousel for in-line leak testing containers.
 63. The apparatus ofclaim 45 , comprising an evaluation unit with at least one pressuresignal input, said evaluation unit comprising at least one comparatorunit, one input thereof being operationally connected to said at leastone pressure signal input, a second input thereof being operationallyconnected to a controllable threshold value unit; an averaging unitcontrollably operationally connected to said input; said evaluation unitgenerating a leak-identifying signal; said leak-identifying signalcontrolling said operational connection of said input and said averagingunit; the output of said averaging unit controlling said controllablethreshold unit.
 64. The use of the method according to one of claims 1to 44 or of the apparatus according to at least one of claims 45 to 63for leak testing blisters, vials, medical application containers,foodstuff or beverage containers, tanks.
 65. The use of the methodaccording to one of claims 1 to 44 or of the apparatus according to oneof claims 45 to 63 for permanently testing tank plants for leakage.